None.
This invention relates to electric power test equipment, more specifically to sag generators that intentionally create power quality disturbances on an alternating current system.
It is often desirable to create power quality disturbances on alternating current systems. Such disturbances can be used, for example, to test the immunity of newly designed systems. For example, the SEMI-F47 standard, published by the industry association Semiconductor Equipment and Materials International, and the associated SEMI-F42-0600 testing standard, require that all semiconductor manufacturing equipment tolerate voltage sags to 50% of nominal for 200 milliseconds, to 70% of nominal for 500 milliseconds, and to 80% of nominal for 1 second.
Devices that generate such sags for testing purposes are known as sag generators.
Sag generators are often used to test large, room-sized systems that cannot be readily transported to another location, such as semiconductor manufacturing systems. The sag generator must generally be transported to the system to be tested. For this reason, a key element in sag generator design is portability.
It is well known to those familiar with the art that dissipating large amounts of heat is difficult in a portable instrument. Consequently, minimizing heat losses in sag generators is a desirable goal.
Transformer-based sag generators are well-known in the art. Grady et al. in U.S. Pat. No. 5,886,429 and Rockfield et al. in U.S. Pat. No. 5,920,132 disclose typical transformer-based sag generators. In a transformer-based sag generator, switching devices are used to change from a non-sagged voltage, typically the input voltage, to a sagged voltage, typically a tap on the transformer. After a brief interval of time, typically limited to a few seconds, the switching devices change back to the non-sagged voltage.
Sag generators are rated by their nominal voltage and their maximum continuous current, for example 480 volts and 100 amps. The characteristics of the switching devices often determine the performance, and set the limits, for the sag generator as a whole. In particular, the heat losses in the switching devices are a critical factor.
These switching devices may be mechanical switches, such as contactors or relays; or solid state switches, such as triacs or insulated gate bipolar transistors. Each type of switching device has its advantages and disadvantages.
Mechanical switching devices, such as contactors or relays, generate minimal heat losses. Typical forward voltage drop across the switch is roughly 0.2 volts, which corresponds to 20 watts of heat at 100 amps. However, this type of switch cannot be easily used to control the phase angle at which the transition to a voltage sag takes place. In addition, this type of switch can introduce unintended transient conditions during transition from one state to another, due to physical movement of the switch contacts.
Triac switching devices generate more heat losses than mechanical switching devices. Typical forward voltage drop across the switch is roughly 1.7 volts, which corresponds to 170 watts of heat at 100 amps. In addition, they limit the phase angle control for sag transitions to 0xc2x0 or 180xc2x0 on the current waveform.
IGBT (insulated gate bipolar transistor) switching devices generate even more heat losses than triac switching devices. Depending on the configuration, the forward voltage drop across the switch can range up to 5 volts, which corresponds to 500 watts of heat at 100 amps. On the other hand, IGBT switching devices permit accurate phase control and minimal switching transitions.
It is an object of this invention to provide a tranformer-based sag generator that combines the advantages of two or more switch technologies, while avoiding the disadvantages of those switch technologies.
It is a further object of this invention to provide a sag generator that allows automatic control of the sag depth, sag duration, sag envelope, and sag phase angle.
Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings.